A portable power tool, such as a chain saw, a hedge trimmer, an edger, or a blower, is often powered by a gasoline engine. Typically, a carburetor is associated with such a smaller engine.
A small engine carburetor has one or more adjustable components, often referred to as needle valves, which control fuel flow. To calibrate fuel flow, each valve is rotationally adjusted to axially extend or retract a valve control surface within a fuel passage in the carburetor until optimum fuel flow through the passage is achieved. Adjustment within the carburetor is commonly done by “metering” during no load or idle engine operation by a first “idle” needle valve and during part or full load operation by a second “main” needle valve. Pre-calibration of the carburetor on a flow test bench may be performed prior to being assembled to an engine. After assembly of the carburetor to an engine, it is customary to adjust the metering valves, if needed, to fine tune fuel flow to the actual demand of the operating engine. Such calibration is first done at an assembly facility, and may subsequently be done by a qualified technician.
Recently, in response to federal and state “clean air” regulations, there has developed an increasing need to prevent gross misadjustment, especially by non-qualified people. Such prevention of gross misadjustment is directed to the prevention of excessive emission of carbon monoxide and unburned hydrocarbons from an internal combustion engine. Thus, subsequent over adjustment of fuel flow is to be prevented/discouraged.